OSPF overview

OSPF (Open Shortest Path First) is perhaps the most popular link state routing protocol. It is an open standard so it can be run on routers produced by different vendors, which is in contrast to EIGRP, a Cisco proprietary protocol that can be run only on Cisco devices.

OSPF is classless and supports features such as VLSM, manual route summarization, incremental updates, equal cost load balancing, etc. As the metric, OSPF uses a single parameter – the interface cost. OSPF uses the multicast addresses of and are for the communication between OSPF-enabled routers. The default administrative distance for OSFP routes is 110.

Routers running OSPF need to establish the neighbor relationship before exchanging routing updates. Since OSPF is a link state routing protocol, neighbors don’t exchange routing tables; instead, they exchange information about network toplogy. Each OSFP router then runs the SFP algorithm to calculate the best routes and adds those to the routing table. Since each router knows the entire topology of a network, a chance for a routing loop to occur is minimal.

OSPF routers stores routing and topology information in three tables:

  • neighbor table – stores information about OSPF neighbors.
  • topology table – stores the topology structure of the network.
  • routing table – stores the best routes.
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